1. Field of the Invention
The present invention relates to an optical cross-connect device with a redundant configuration, and in particular, relates to an optical cross-connect device with a redundant configuration that is used in a wavelength-division multiplex optical communications system.
2. Description of the Related Art
With the advent of high-speed/large-capacity data transmission, broadband/large-capacity networks and communication devices are in high demand. In such a situation, the construction of an optical network based on a WDM (wavelength-division multiplexing) technology is desired as one method for realizing the broadband/large capacity networks and communications devices. A core device in constructing such an optical network is an optical cross-connect device. In the following description, the optical cross-connect device is sometimes called an “optical XC”.
FIG. 1 shows a network environment in which an optical cross-connect device is used. In this case, this optical network comprises a plurality of optical cross-connect devices connected to one another through an optical transmission line. WDM light is transmitted through an optical transmission line connecting the optical cross-connect devices. WDM light includes a plurality of wavelengths λ1 through λn.
An optical cross-connect device 500 accommodates a plurality of input-side optical inter-station transmission lines and a plurality of output-side optical inter-station transmission lines. Here, WDM signal light is transmitted through each inter-station transmission line. The optical cross-connect device 500 guides WDM signal light received through the input-side optical inter-station transmission line to a designated output-side optical inter-station transmission line for each destination or wavelength. If the distance between the optical cross-connect devices is long, one or more optical amplifiers are inserted in the optical inter-station transmission line. The optical cross-connect device 500 is also often connected to a switch (for example, electric cross-connect device, etc.) accommodating subscriber lines. Then, the optical cross-connect device 500 is controlled by an operating system managing the entire network.
The number of wavelengths of WDM signal light transmitted between the optical cross-connect devices in the optical network has been increasing every year. Therefore, if the optical cross-connect device fails, its influence on communication services becomes very serious. For this reason, the optical cross-connect device is redundantly configured so as to promptly recover from a failure that occurs inside the device.
FIG. 2 shows the configuration of an existing optical cross-connect device. This optical cross-connect device comprises one set of switches (switch units 501-W(0) and 501-P(1)). The optical inter-station transmission path of an optical network in which this optical cross-connect device is used is also duplicated. Specifically, one set of optical inter-station transmission lines (systems 0 and system 1) through which signals are transmitted from one optical cross-connect device to another, and one set of optical intra-station transmission lines (systems 0 and system 1) through which signals are transmitted in the opposite direction are installed between every pair of optical cross-connect devices. In this example, it is assumed that identical signals are transmitted through one set of optical inter-station transmission lines. That is to say, the signal transmitted through one of the set of optical inter-station transmission lines is the same as the signal transmitted through the other one of the set of optical inter-station transmission lines.
WDM signal light (λ1 through λn) received through one set of the optical inter-station transmission lines (systems 0 and system 1) is amplified by an optical amplifier 502 and is demultiplexed for each wavelength by a demultiplexer 503. Each piece of signal light demultiplexed for each wavelength is branched into one set of signal light by an optical coupler 504, which is used as an optical splitter. One piece of signal light split by the optical coupler 504 is sent to a 2×1 switch 505 installed on the input side of the switch unit 501-W(0), and the other piece of signal light is sent to a 2×1 switch 505 installed on the input side of the switch unit 501-P(1). Thus, both of the signal light received through system-0 optical inter-station transmission line and the signal light received through system-1 optical inter-station transmission line are input to each 2×1 switch 505.
The 2×1 switch 505 selects one piece of signal light and outputs the signal to the corresponding switch units (501-W(0), 501-P(1)). Then, each of the switch units 501-W(0) and 501-P(1) performs routing processes on the incoming signal light according to the instructions of the operating system.
Each piece of signal light output from the switch units 501-W(0) or 501-P(1) is split into one set of signal light by an optical coupler 506 used as an optical splitter. One piece of signal light split by the optical coupler 506 is sent to a 2×1 switch 507 corresponding to the system-0 optical inter-station transmission line, and the other piece of signal light is sent to a 2×1 switch 507 corresponding to the system-1 optical inter-station transmission line. Thus, both of the signal light guided by the switch unit 501-W(0) and the signal light guided by the switch unit 501-P(1) are input to each 2×1 switch 507.
The 2×1 switch 507 selects and outputs one piece of signal light. The signal light output from the 2×1 switch 507 is regenerated by an optical regenerator 508. Then, the signal light regenerated for each wavelength is multiplexed by a multiplexer 509 and is output to a corresponding optical inter-station transmission line as WDM signal light. At this time, the WDM signal light is amplified by an optical amplifier 510.
As described above, in the existing optical cross-connect device, signal light received through an optical inter-station transmission line is guided to both of the switch units 501-W(0) and 501-P(1). Then, one of the two pieces of signal light individually routed by the switch units 501-W(0) and 501-P(1) is selected and guided to an output-side optical inter-station transmission line. In other words, the path established inside the optical cross-connect device is duplicated. Therefore, if one path cannot be used due to the failure of an optical parts in the optical cross-connect device, the device can promptly recover from the failure by using the other path.
However, in the existing optical cross-connect device shown in FIG. 2, the optical regenerator 508 is provided for each output-side optical inter-station transmission line. Here, as shown in FIG. 2, the optical regenerator 508 is provided for each wavelength. Since in recent WDM optical transmission systems, the number of wavelengths multiplexed has been increased, the number of optical regenerators 508 needed for each optical inter-station transmission line has also been increasing proportionally. Furthermore, generally the optical regenerator 508 is fairly expensive. As a result, the size and cost of the optical cross-connect device have been increasing.
In addition, in the existing optical cross-connect device shown in FIG. 2, two optical couplers and two selectors (2×1 switches) are provided in the path from an input port to an output port. For this reason, the optical loss inside the optical cross-connect device becomes large. Therefore, requirements on an optical amplifier (gain, etc.) become severe.